InternetAddressing 11A-3
Octel 200/300 S.4.1PB60019−01
all 0s
* Allowed only at system startup and is never a valid destination address.
** Never a valid source address.
*** Should never appear on a network.
This host*
all 0s host Host on this net*
ident.
class
all 1s Limited broadcast (local net)**
net all 1s Directed broadcast for net**
127 anything (often 1) Loopback***
Note: The length of the net portion depends on its class.
Special Forms of IP Addresses
Subnet Addressing
The netid portion of an IP address is assigned to an organization by a central agency called IANA.[The
individual organization is free to divide its hostid portion into multiple smaller physical networks called
subnets.
Suppose a site has a class B address. It has 16 bits of the address to use for its local portion. Suppose
there are eight physical networks at the site. For purposes of addressing efficiency,the organization could
take the top 3 bits from its local portion and assign one value to each of its eight physical networks. This
particular combination gives eight physical networks (subnets) with up to 8,192 hosts per subnetwork.
Another site may choose to assign 8 bits to the subnet, giving 256 subnets each with up to 256 hosts.
Refer to the following figure.
To allow maximum autonomy,the TCP/IP subnet standard allows the subnet partition to be selected on a
per-network basis. Once a partition has been selected for a particular network, machines attached to that
network are expected to use it.
The subnet mask is usually given to you by your LAN administrator along with your system’s IP address.
If your network doesn’t use a subnetting scheme, you should still set up the subnet mask to cover your
network portion. Refer to Example 3 in this chapter.
[For information on how to get your own assigned IP address, contact Joyce K. Reynolds, (213) 882-1511 JKREY @
ISI.EDU Internet Assigned Numbers. Authority, USC-Information Sciences Institute, 4676 Admiralty Way,Marina
del Rey, California 90292-6695.